1. Field of the Invention
The present invention relates to hydrokinetic coupling apparatus, especially for a motor vehicle.
2. Description of the Related Art
Numerous hydrokinetic coupling apparatuses are known in the current state of the art, among which are various designs of apparatus according to their applications, and in particular apparatus of the “monoface”, “biface” or again “multi-disc” types.
It is for example known, from the documents FR-A-2 825 770 or FR-A-2 765 938, to provide a hydrokinetic coupling apparatus of the “monoface” type, which comprises, considered axially from front to rear:                a casing consisting of a rear shell which is adapted to be coupled in rotation to a driving shaft, an impulse wheel, and a front shell;        a turbine wheel, which is arranged for rotation with a turbine hub, which is adapted to be coupled in rotation to a driven shaft;        a lock-up clutch for coupling the driving shaft and the driven shaft together, which is operatively interposed between the turbine wheel and the rear shell and comprises a piston, which is movable axially for releasably coupling together the rear shell and the driven shaft), and which includes a damping device, the damping device comprising at least one guide ring which constitutes the input element, a damper plate constituting the output element, and circumferentially acting elastic members interposed between the input element and output element, which are coupled together in rotation but with the ability to perform predetermined circumferential displacement,        and of the type in which the turbine wheel, the turbine hub and the damper plate of the damping device are coupled in rotation by means of joints that are rigid, i.e. they have no play.        
In general terms, the rigid rotational couplings between the turbine wheel and turbine hub or an input or output element of the damping device, which is also called the damper, can be achieved with any appropriate kind of joint, and especially by riveting, welding or meshing engagement.
Thus, there are generally, in particular, a first rigid joint by means of which the turbine wheel is coupled in rotation to the turbine hub, and a second rigid joint by means of which the turbine wheel is coupled in rotation to an element of the damper.
In the document FR-A-2 825 770, the hydrokinetic coupling apparatus of the “monoface” type includes a first coupling between the turbine wheel and the turbine hub, which is formed by riveting, and a second coupling between the turbine wheel and the damper plate which is formed by welding.
In a modified version in the document FR-A-2 765 938, the second coupling between the turbine hub and the damper plate of the damper is also made by riveting.
Hydrokinetic coupling apparatuses of the “multi-disc” type are also known, in which the coupling means between the turbine wheel, turbine hub and input element of the damping device are axial rivets, and such an arrangement is for example described in the document FR-A-2 839 128.
However, riveting is a method of making a joint which does not always give full satisfaction. Riveting has various disadvantages, and in particular it requires costly machining operations by way of precision drilling in each of the components to be coupled together in rotation, to form the holes for the passage of the bodies of the rivets.
In addition, the heads of the rivets increase the general axial size of the apparatus, and although riveting is simple to apply, it is an expensive way of fastening because of the overall time needed to carry out all the various operations.
The document U.S. Pat. No. 5,975,261 describes a further hydrokinetic coupling apparatus of the “multi-disc” type, in which a first rigid coupling is made by friction welding, and the second coupling is made by mating cooperation between, on the one hand, lugs which are formed in the guide ring, and secondly apertures provided in the turbine hub.
Rigid couplings of the meshed type also have disadvantages, in particular as regards treatments and precision machining operations in order to make the complementary male and female parts, and they are therefore expensive. In addition, a coupling of this kind is not reliable enough in respect of its liability to wear, so that undesirable noise will tend to occur.
That is why welding, and more particularly friction welding, is often the preferred fastening method for coupling the turbine wheel, turbine hub and damper plate, or one of the guide rings of the damper, rigidly together.
However, with welding it is also necessary that the weld bands be accessible, in particular in order to enable operations of control and cleaning of the weld bands to be carried out.
Nevertheless, in currently known apparatus designs, the turbine wheel, turbine hub and damper plate do not enable operations of these kinds to be easily performed, and therefore do not give any guarantee as to the quality and reliability of the welded joints.
In addition, in current motor vehicles, the space available for fitting the engine is becoming smaller and smaller, especially because of the addition of further components, so that smaller size, and consequently a high degree of axial compactness, of the apparatuses, is constantly being sought.
More precisely, in the case where the turbine hub is interposed axially between the turbine wheel and the damper plate of the damper, it is necessary to control the first welded joint, at the front, which couples the turbine wheel with the turbine hub in rotation, and also the second joint at the rear which couples the hub rigidly to the damper plate of the damper.
Now, those designs which are currently known do not enable access to be gained satisfactorily to the weld bands of the first and second joints, and particularly to the inner rear weld band of the second joint.